1. Field of the Invention
The invention relates to a rotor for an electric motor (interior permanent magnet (IPM) motor) which includes permanent magnets inserted within slots that are formed in a rotor core, and to a production method for the rotor.
2. Description of Related Art
Among various electric motors, such as brushless DC motors, there is a widely known motor that has an embedded-permanent magnet type rotor in which a plurality of permanent magnets are embedded within a rotor core (hereinafter, referred to as “IPM motor”). The IPM motors are used as, for example, motors for hybrid vehicles.
In electric motor manufacturers that manufacture IPM motors (including motor vehicle manufactures), permanent magnets to be embedded in rotors are obtained from magnet manufactures, and are inserted and fixed in magnet slots that are formed in rotor cores to manufacture rotors.
The permanent magnets obtained from a magnet manufacturer have product errors (product tolerances) as a matter of course. This will be explained simply on a weight basis. There are cases where, for permanent magnets of a weight of X (g), a product error (weight error) of, for example, ±0.01X (g), exists.
Manners of arrangement of permanent magnets in a rotor core vary; for example, there are a configuration in which one magnetic pole is formed by one permanent magnet, or a configuration as disclosed in Japanese Patent Application Publication No. 2005-57958 (JP-A-2005-57958) in which one magnetic pole is formed by disposing two permanent magnets in, for example, a V shape in a plan view. Regarding the permanent magnet configuration of the aforementioned V arrangement, which, with the flow of magnetic fluxes from the stator side to the rotor side taken into account, is able to achieve greater efficiency and higher reluctance torque, if the two permanent magnets of each magnetic pole have individual product errors as mentioned above, the product error per magnetic pole is twice the product error of one permanent magnet (in the foregoing example, the product error (weight error) per magnetic pole is ±0.02X (g)).
Besides, the aforementioned doubled product error brings about weight imbalance of the rotor, and this imbalance has been found to lead directly to variation and reduction of the motor torque, by the inventors. Generally, a rotor is equipped with a drive shaft attached to a center opening of the rotor, and the drive shaft is rotatably retained by bearing gears outside the motor.
Let us take as an example an IPM motor rotor in which eight magnetic poles each of which is formed by two permanent magnets arranged in a V shape (hence, 16 permanent magnets in total) are formed in the circumferential direction of the rotor core. This rotor can have a weight error as great as ±0.16X (g), which is a total weight error of the permanent magnets. The weight errors of the permanent magnets of the individual magnetic poles can give an excessive eccentric load to the bearing gears that rotatably retain the drive shaft, and therefore can reduce the durability of the gear. Or, the weight errors of the permanent magnets result in different centrifugal forces acting on the individual magnetic poles when the rotor rotates, which can cause vibration or noise to be produced between the rotor and the drive shaft. In a more detailed discussion on the centrifugal force, since the centrifugal force depends on both the weight of the permanent magnets and the distance from the rotor center to the position at which the permanent magnets are disposed, it is naturally necessary to secure fine control not only on the weight of the permanent magnets but also over the process of defining the positions of the slots for the permanent magnets.
Therefore, in an actual electric motor manufacturer, an adjustment process is sometimes performed in which when the two opposite end portions of the rotor core are capped with end plates after a predetermined number of permanent magnets are inserted and fixed within the rotor core, the weight imbalance of the permanent magnets of each magnetic pole is lessened in the entire rotor by, for example, forming reduced-wall thickness portions at suitable locations on the end plates, so as to adjust the weight imbalance of the permanent magnets of each magnetic pole.
However, it is easily understandable that such a weight adjustment requires a large number of workers and a large amount of time. The problem of an increased manufacture time resulting from the weight adjustment needs to be promptly solved, for example, in the case where the rotors produced are applied to drive motors of hybrid vehicles, whose production is recently becoming larger and larger.